1. Field of the Invention
The invention relates to reversibly reactivatable dual status markers useful in magnetic-type electronic article surveillance (EAS) systems. When an article bearing one of those markers is passed through an interrogation zone of alternating magnetic fields, the magnetization state of the marker is periodically reversed, and a remotely detectable characteristic response (here sometimes called "an alarm signal") is produced. The invention also relates to the manufacture of such markers and to a product that can be used as a control element to afford dual status to a marker of the prior art.
2. Description of the Related Art
Magnetic-type EAS systems are widely used to inhibit the theft of merchandise such as clothing, books, and cassettes. Markers used in such systems typically have comprised elongated ribbons of metal foil that exhibit high permeability and low coercive force to enable their state of magnetization to reverse in the relatively low intensity alternating magnetic fields typically associated with magnetic-type EAS systems. Those fields have average peak intensities of a few Oersteds, typically ranging from about one Oersted at the center to about 20 Oersteds at the edges of the interrogation zone of an EAS system.
In coassigned U.S. Pat. No. 3,765,007 (Elder), a remanently magnetizable layer is laminated to such a ribbon and acts as a control element to afford dual status to the marker. It has a passive status when the remanently magnetizable layer is magnetized to prevent the marker from producing an alarm signal when an article to which it is attached is carried through an interrogation zone of an EAS system. It has a sensitized status when the remanently magnetizable layer is demagnetized, thus enabling the marker to produce an alarm signal. The Elder patent also suggests that a disc-shaped marker, which is sufficiently thin, can have a useful demagnetization factor.
Coassigned U.S. Pat. Nos. 4,710,754 and 4,746,908 (both Montean) disclose markers of low coercive force, high permeability material such as permalloy foil that can be the size of postage stamps. The foil of Montean '754 is shaped to have at least one switching section and flux collectors proximate to each end of each switching section. When it has at least two switching sections that extend in substantially different directions, the marker can be detected regardless of its orientation. Such a marker can be called "bi-directional" in contrast to the unidirectional markers of the Elder patent. When a sensitized unidirectional marker is passed through an interrogation zone with its easy axis perpendicular to all components of the alternating magnetic fields, the magnetization of its signal-producing foil might not be reversed and thus fail to produce an alarm signal.
The marker of Montean '908 also can be bi-directional. It has a specially shaped foil which bears a remanently magnetizable layer that, when magnetized in a predetermined pattern, permits the marker to produce an alarm signal in an interrogation zone. When the remanently magnetizable layer is demagnetized, the marker will not produce an alarm signal.
Markers of Montean '754 and '908 are currently being marketed as QuadraTag.TM. EAS markers by the company to which this application is assigned.
U.S. Pat. No. 4,960,651 (Pettigrew et al.) points out that a marker, when sensitized, should have low demagnetization factors to permit low intensity interrogation fields to be used, and that metal ribbons such as ribbons of permalloy foil must be quite long to achieve low demagnetization factors. The Pettigrew patent concerns a marker that has low demagnetization factors and is made by depositing onto a substrate a magnetic thin-film of low coercive force and high permeability that preferably is from 1 to 5 .mu.m in thickness. As compared to markers of metal ribbons which are relatively thick (generally over 10 .mu.m and often about 25 .mu.m in thickness), the Pettigrew thin-film marker can be quite thin and more mechanically flexible and hence more robust. By being thin, it can be less conspicuous. Also, it can have a more convenient shape such as the dimension and shape of a price label and can be square or circular.
The Pettigrew marker can be made to have dual status by applying a deactivation layer of semi-hard magnetic material to change the effective magnetic properties of the magnetic thin-film of low coercive force so that it is not recognized in an interrogation zone. In each of the examples of the Pettigrew patent, the deactivation layer employed a thin sheet (col. 15, line 9) or a foil, slurry, needles, or steel wool or mesh (col. 16, lines 15-24). The Pettigrew patent does suggest: "The deactivating material may be fabricated by thin film processes" (col. 6, lines 62-64) but includes no enabling disclosure, instead referring to disclosure for fabricating the magnetic thin-films of low coercivity.
Still others have sought to provide markers utilizing thin-films. Thus, for example, Fearon, U.S. Pat. No. 4,539,558 (col. 16, lines 2-14), has proposed that an elongated marker may be formed of a strip of alternating sputtered layers of ferromagnetic materials. In that construction, each layer is separated by an evaporated coating of, for example, aluminum oxide. Fearon still emphasizes the necessity of an elongated shape and the subsequent need for appropriate orientation in an interrogation field. See also U.S. Pat. No. 4,682,154 (Fearon).
The marker of coassigned U.S. Pat. No. 5,083,112 (Piotrowski et al.) comprises a laminate of a plurality of magnetic thin-films deposited on a flexible substrate with an ultrathin nonmagnetic thin-film interposed between adjacent magnetic thin-films. Each of the magnetic thin-films exhibits high permeability and a coercive force sufficiently low so as not to retain any given magnetization state and less than the average intensity of magnetic fields encountered in an interrogation zone, such that upon exposure to such fields, the magnetization state of the marker is periodically reversed and an alarm signal is produced. When the easy axis of one of the magnetic thin-films extends in a direction different from that of another (as in FIG. 3) or a magnetic thin-film has more than one easy axis, the marker is bi-directional.
The Piotrowski patent demonstrates that a plurality of magnetic thin-films permit the markers to be smaller and yet produce sharper, more intense signals than was possible in the prior art. Its five examples employ from 7 to 15 pairs of magnetic thin-films (e.g., Ni-Fe) and nonmagnetic thin-films (e.g., SiO.sub.x). To provide dual status, the Piotrowski marker can include a layer of remanently magnetizable material such as a thin foil of magnetic stainless steel or vicalloy or a dispersion of gamma iron oxide particles.